Modern digital communications systems require accurate and stable reference signals for use in modulation and demodulation of transmitted signals. For example, digital communications receivers typically contain a carrier recovery loop that is used to generate a carrier signal from the received transmission. FIG. 1 depicts a block diagram of an illustrative carrier recovery loop 100. Within the loop is a multiplier 102, a phase detector 104, a loop filter 106, and a voltage controlled oscillator (VCO) 108. The purpose of the carrier recovery loop is to lock the phase and frequency of the signals generated by the VCO (a carrier) to the received signal. To this end, the VCO generates a sinewave signal (carrier) that is typically phase locked to the received signal. In an M-ary, quadrature digital communications receiver, the VCO typically generates both a sine and a cosine waveform to phase lock to both I and Q-channel baseband (or near baseband) signals. In operation, the I and Q channel signals (on path 101) are phase detected in phase detector 104 to produce a phase error signal and loop filter 106 extracts the DC component of the phase error signal. The DC component is then applied to the VCO 108. The DC component adjusts the phase and/or frequency of the VCO to attain frequency lock with the received signal. The VCO generates both a cosine and sine waveform that are multiplied with the received signal at multiplier 102.
More specifically, the VCO comprises digital components including a modulo integrator 110 and at least one look-up table 112 and/or 114. The control signal of the VCO is an error voltage, e(t), typically generated by a low pass filtered signal from the phase detector 104. The DC level of the control signal is indicative of the phase error between the received signal and the VCO's signal. The modulo integrator includes an adder 116, a modulo function 118, and a one symbol delay 120. The input signals to the adder are the control signal and the output signal of the delay. The output of the adder forms the input to the modulo function and the output of the modulo function forms an input to the delay. This modulo integrator accumulates the control signal (error voltage) and "wraps" at some predefined overflow level of accumulation such that, when the control signal is a constant DC value, the integrator produces a sawtooth waveform at its output.
The instantaneous value of the output waveform of the integrator forms an input value (memory address) to one or more look-up tables 112, 114. Typically, in an M-ary receiver, there is a sine value look-up table 114 and a cosine value look-up table 112. Thus for each value of integrator waveform, the look-up tables generate a value of a sine function and a value of a cosine function. These values, when taken sequentially, form a sine waveform and a cosine waveform that are used by the receiver to demodulate the received transmission.
The size of the look-up tables, i.e., the number of values available to be referenced, is directly related to the desired precision of the sine and cosine waveforms. In other words, a very smooth waveform can only be generated if many, many values are available for recall from the tables. Generally, the look-up table technique for synthesizing waveforms is used for relatively simple digital communications systems, e.g., 4-ary or 16-ary quadrature amplitude modulation (QAM) techniques, where imprecise reference waveforms are sufficient. However, for modulation systems using 64-ary or 256-ary QAM, the reference waveforms generated by the VCO must be well-defined, precise and smooth. As such, the look-up tables can become excessively large and, as such, costly.
U.S. Pat. No. 4,285,044, issued Aug. 18, 1981, discloses an example of a numerical sinewave generator, i.e., a generator that does not utilize a look-up table. This generator uses a numerical technique to synthesize a sinewave given a constant phase value. Specifically, the generator has two input values, the cosine and sine of a constant phase angle, and synthesizes a sinewave of a fixed frequency and phase from the two input values. This patent is important for its disclosure of a numerical technique for synthesizing a sinewave; however, the generator lacks any capability of being voltage controlled. Thus, such a fixed frequency generator is not useful for modern digital communications systems.
Therefore, a need exists in the art for a numerical voltage controlled oscillator having an adjustable frequency and phase output signal and that does not use look-up tables to generate sine and cosine waveforms.